An electronic amplifier accepts as its input an electronic signal and produces as its output a stronger version of that electronic signal. For example, recording an electrocardiogram on a chart requires amplifying the weak electrical signal produced by a beating heart until the signal is strong enough to move a pen up and down as a paper chart moves past the pen.
A linear amplifier is one in which there is a linear relationship between the electronic signal it receives as input and the electronic signal it produces as output. That is, for a change of X units in its input voltage or current, it produces a change in its output voltage or current of k*X (k times X) units for some constant value k, regardless of whether the value of the input signal is small or large.
Every electronic circuit is unable to product outputs larger than some limit. Every electronic circuit is unable to effectively handle inputs larger than some limit or smaller than some other limit. Nevertheless, for many applications of electronic circuits, it is necessary that they be operated only within a middle range where they produce a linear response to changes in their inputs.
Common examples of practical applications where linear amplifiers are used include both audio amplifiers and radio-frequency amplifiers. If you listen to an audio amplifier that is not linear in its response, then you hear the music or voice as distorted or flat.
Non-linear responses in radio-frequency amplifiers can produce cross talk or intermodulation between the desired signal and another extraneous radio signal that happens to be present at the same time, but on a different frequency or channel. Such undesired signals are called jamming sources whether or not the interference is intentional. When an amplifier behaves non-linearly, for example, a change of X in its input signal produces less than k*X change in its output signal, then the effect of this non-linearity is to shift the frequency of the signal that it amplifies. If a desired signal and a jamming source at different frequencies are present at the same time (which Is typical of the operating environment for radio receivers), then this frequency shift results in cross talk or intermodulation between the two signals.
Many electronic amplifiers electrically combine their input signal with a constant or bias voltage or current. The amount of bias used is chosen in order to set an appropriate operating point for the amplifier. When an electronic amplifier is designed, an important choice is whether to make that constant bias have a relatively large or a relatively small value. The bias value chosen when designing the amplifier can have major consequences on how and how well it operates.
One standard technique in designing a linear amplifier is to first specify the range of the input signal over which the amplifier must respond linearly and the degree to which the amplifier must reject intermodulation from undesired sources. Then, the amount of bias current or voltage is set so as to meet to these specifications. The larger the range of linearity desired and the lower the amount of intermodulation that is acceptable, then the larger the bias must be.
Unfortunately, the larger the bias of an amplifier, the more power it consumes. Thus, there is a tradeoff between an amplifier's power consumption on the one hand and its range of linearity and susceptibility to intermodulation on the other hand. The design goal of minimizing power consumption opposes the design goal of maintaining acceptable linearity.
Power conservation is always desirable. But with the advent of widely used mobile, hand-held and pocket wireless devices, such as pagers and cellular telephones, its importance has increased.
The radio-frequency amplifiers, buffers and other front-end circuitry in a pager or in the receiver section of a cellular or other mobile telephone must be operating in order for the device to respond to a page or phone call broadcast to it. Thus, the length of time that a battery will last while the device is standing by for a page or a phone call depends on how much power is consumed by its receiver. To many consumers, most of the power consumed by the device is consumed in standby mode—for example, a mobile phone may be standing by for a call many hours each day but in use for calls only minutes each day.
Longer battery life reduces the costs and increases the convenience for consumers who use, for example, portable devices, including but not limited to mobile devices, hand held devices, pagers, mobile phones, digital phones, PCS phones and AMPS phones. In these highly competitive markets, battery life in standby mode can make the difference as to which competing product the consumer chooses. Thus, it is critical for the market success of mobile, portable and hand-held receivers that they consume a minimum of power, particularly in standby mode.
The standby battery life of a mobile receiver can be significantly increased by lowering its power consumption by lowering the bias level used in its front-end circuits such as amplifiers and buffers. However, prior art techniques for doing this also reduce the receivers linear range and thus increase its susceptibility to intermodulation from jamming sources.